Below is the syntax highlighted version of Inversions.java.
/****************************************************************************** * Compilation: javac Inversions.java * Execution: java Inversions < input.txt * Dependencies: StdIn.java StdOut.java * * Read array of n integers and count number of inversions in n log n time. * ******************************************************************************/ package edu.princeton.cs.algs4; /** * The {@code Inversions} class provides static methods to count the * number of <em>inversions</em> in either an array of integers or comparables. * An inversion in an array {@code a[]} is a pair of indicies {@code i} and * {@code j} such that {@code i < j} and {@code a[i] > a[j]}. * <p> * This implementation uses a generalization of mergesort. The <em>count</em> * operation takes Θ(<em>n</em> log <em>n</em>) time to count the * number of inversions in any array of length <em>n</em> (assuming * comparisons take constant time). * <p> * For additional documentation, see * <a href="https://algs4.cs.princeton.edu/22mergesort">Section 2.2</a> * of <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne. * * @author Robert Sedgewick * @author Kevin Wayne */ public class Inversions { // do not instantiate private Inversions() { } // merge and count private static long merge(int[] a, int[] aux, int lo, int mid, int hi) { long inversions = 0; // copy to aux[] for (int k = lo; k <= hi; k++) { aux[k] = a[k]; } // merge back to a[] int i = lo, j = mid+1; for (int k = lo; k <= hi; k++) { if (i > mid) a[k] = aux[j++]; else if (j > hi) a[k] = aux[i++]; else if (aux[j] < aux[i]) { a[k] = aux[j++]; inversions += (mid - i + 1); } else a[k] = aux[i++]; } return inversions; } // return the number of inversions in the subarray b[lo..hi] // side effect b[lo..hi] is rearranged in ascending order private static long count(int[] a, int[] b, int[] aux, int lo, int hi) { long inversions = 0; if (hi <= lo) return 0; int mid = lo + (hi - lo) / 2; inversions += count(a, b, aux, lo, mid); inversions += count(a, b, aux, mid+1, hi); inversions += merge(b, aux, lo, mid, hi); assert inversions == brute(a, lo, hi); return inversions; } /** * Returns the number of inversions in the integer array. * The argument array is not modified. * @param a the array * @return the number of inversions in the array. An inversion is a pair of * indicies {@code i} and {@code j} such that {@code i < j} * and {@code a[i] > a[j]}. */ public static long count(int[] a) { int[] b = new int[a.length]; int[] aux = new int[a.length]; for (int i = 0; i < a.length; i++) b[i] = a[i]; long inversions = count(a, b, aux, 0, a.length - 1); return inversions; } // merge and count (Comparable version) private static <Key extends Comparable<Key>> long merge(Key[] a, Key[] aux, int lo, int mid, int hi) { long inversions = 0; // copy to aux[] for (int k = lo; k <= hi; k++) { aux[k] = a[k]; } // merge back to a[] int i = lo, j = mid+1; for (int k = lo; k <= hi; k++) { if (i > mid) a[k] = aux[j++]; else if (j > hi) a[k] = aux[i++]; else if (less(aux[j], aux[i])) { a[k] = aux[j++]; inversions += (mid - i + 1); } else a[k] = aux[i++]; } return inversions; } // return the number of inversions in the subarray b[lo..hi] // side effect b[lo..hi] is rearranged in ascending order private static <Key extends Comparable<Key>> long count(Key[] a, Key[] b, Key[] aux, int lo, int hi) { long inversions = 0; if (hi <= lo) return 0; int mid = lo + (hi - lo) / 2; inversions += count(a, b, aux, lo, mid); inversions += count(a, b, aux, mid+1, hi); inversions += merge(b, aux, lo, mid, hi); assert inversions == brute(a, lo, hi); return inversions; } /** * Returns the number of inversions in the comparable array. * The argument array is not modified. * @param a the array * @param <Key> the inferred type of the elements in the array * @return the number of inversions in the array. An inversion is a pair of * indicies {@code i} and {@code j} such that {@code i < j} * and {@code a[i].compareTo(a[j]) > 0}. */ public static <Key extends Comparable<Key>> long count(Key[] a) { Key[] b = a.clone(); Key[] aux = a.clone(); long inversions = count(a, b, aux, 0, a.length - 1); return inversions; } // is v < w ? private static <Key extends Comparable<Key>> boolean less(Key v, Key w) { return (v.compareTo(w) < 0); } // count number of inversions in a[lo..hi] via brute force (for debugging only) private static <Key extends Comparable<Key>> long brute(Key[] a, int lo, int hi) { long inversions = 0; for (int i = lo; i <= hi; i++) for (int j = i + 1; j <= hi; j++) if (less(a[j], a[i])) inversions++; return inversions; } // count number of inversions in a[lo..hi] via brute force (for debugging only) private static long brute(int[] a, int lo, int hi) { long inversions = 0; for (int i = lo; i <= hi; i++) for (int j = i + 1; j <= hi; j++) if (a[j] < a[i]) inversions++; return inversions; } /** * Reads a sequence of integers from standard input and * prints the number of inversions to standard output. * * @param args the command-line arguments */ public static void main(String[] args) { int[] a = StdIn.readAllInts(); int n = a.length; Integer[] b = new Integer[n]; for (int i = 0; i < n; i++) b[i] = a[i]; StdOut.println(Inversions.count(a)); StdOut.println(Inversions.count(b)); } } /****************************************************************************** * Copyright 2002-2022, Robert Sedgewick and Kevin Wayne. * * This file is part of algs4.jar, which accompanies the textbook * * Algorithms, 4th edition by Robert Sedgewick and Kevin Wayne, * Addison-Wesley Professional, 2011, ISBN 0-321-57351-X. * http://algs4.cs.princeton.edu * * * algs4.jar is free software: you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation, either version 3 of the License, or * (at your option) any later version. * * algs4.jar is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with algs4.jar. If not, see http://www.gnu.org/licenses. ******************************************************************************/